In a scan by an X-ray CT apparatus, localizer scans for collecting localizer images (scano images) are conducted prior to the main scan. Further, these scans by the X-ray CT apparatus are subject to, for example, AEC (auto exposure control) for calculating the value of tube current to be supplied to an X-ray tube in the main scan, based on the collected localizer images.
A wide scanning range can be set to capture a localizer image because of recent increases in the number of arrayed detectors and the speed of couch movement. However, the pixel dimension of a localizer image varies according to the scanning range of the localizer image. This causes, for example, degradation of a localizer image scanned in a wide range, and may cause an error in the value of tube current calculated by AEC.